Explanatory Notes, <Sx. By Prof. W. A, Rogers, 17 



glass is rubbed as before. In this case one edge of the undisturbed 

 line is sharply defined and the same edge is fairly well defined after 

 cleaning. The strand-like appearance of a few of the single lines 

 first appeared several weeks after the lines were ruled. The dia- 

 mond may now be said to be in a fairly good working condition. 



No. 8. In order to be sure that the diamond with the same set- 

 ting as in No. 7 would continue to rule good lines, the lines of 

 this plate were ruled with a varying pressure and with both back- 

 ward and forward motion of the diamond. The lines of this plate 

 will bear careful study. 



No. 9. In this plate, which immediately followed No. 8, I 

 have for the first time succeeded in preserving positive proof of a 

 fact long suspected, that the test heavy lines are not grooves in the 

 glass but windrows of particles of glass thrown up by the diamond, 

 but so fine that the Microscope cannot separate them. When first 

 ruled, the lines of this plate were of the most beautiful character. 

 After a while the single lines began to show indications of breaking 

 up, while the lines of the band remained nearly intact. As a test, 

 I removed the cover from the cell and rubbed the surface of the 

 glass sharply, at right angles to the lines, but leaving about one- 

 half of the lines undisturbed. Noiu, we have in the upper part of 

 the band the original lines, retaining, it is true, only a portion of 

 their former beauty, but clearly unlike those in which the particles 

 of glass have been, by rubbing, scattered over the entire surface of 

 the groove. The particles stick to the groove with great tenacity. 

 It is impossible to remove them by rubbing crosswise. I shall 

 show in the fourth series of slides that they can be removed by 

 rubbing lengthwise. It will be seen that the undisturbed lines are 



to a certain extent broken up, appearing thus , There 



was no evidence of this appearance for several weeks after the lines 

 were ruled. It is my opinion that the appearance is due to the 

 slight sweating which has taken place. At least I have seen 

 several instances in which this particular appearance is shown in 

 plates in which sweating has taken place. This plate will repay a 

 careful study. 



No. 10. Position of diamond changed a trifle from that with 

 which No. 9 was ruled. 



No. 11. Lines from forward motion of diamond very good. In 

 this plate we have an illustration of what often occurs, namely the 

 formation of minute specks several weeks after the ruling of the 

 plate. I am inclined to believe that they are particles of glass. I 

 am sure that the glass was perfectly clean when mounted. 



No. 12. A slight elevation of diamond after ruling No 11. 

 This is another illustration of the fact stated under No. 9. It is 

 one of the most remarkable specimens I have ever obtained. The 

 plate will repay a most careful study. 



Ser. 2.— Vol. VI. C 



